Fuel



Dec. 3, 1957 E. A. LAURENCE ET AL FUEL Filed July 11, 1945 ELECT/70 MAM/E776 VALVE NI TROMETHANE IV/ 7' ROPROPANE INVENTOR. A

EDGAR LAURENCE BY FRANK L. vonBREGHT A TTOR/VEYS $15 ,276 Patented Dec. 3, 1957 FUEL Edgar A. Laurence, Pasadena, and Frank L. von Brecht, Monrovia, Califi, assignors, by mesne assignments, to Aerojet-General Corporation, Azusa, Calif., 11 corporation of Ohio Application July 11, 1945, Serial No. 604,362

3 Claims. (Cl. 52-.5)

This invention relates to fuels and provides self-combustible fuels which may be employed in internal combustion engines to produce wider limits of application than is obtainable from non-oxidized fuels such as gasoline and to operate with greater safety than is obtainable with previously known monopropellant fuels.

Self-combustible fuels have been employed to operate internal combustion engines of various types, but such fuels usually are limited in application due to their tendency of being sensitive to shock when exposed to the conditions which exist within the operating motor, particularly in motors of the jet propulsion type. For example, a self-combustible fuel such as nitromethane, when subjected to the high temperatures and pressures which exist within the chamber of a jet propulsion motor, tends to become sensitive to shock and sometimes detonates explosively with the attending danger that the explosion will propagate to the main storage supply.

This invention overcomes the foregoing difiiculties by providing fuels that are capable of supporting their own combustion Without the addition of outside oxidizers, and are also relatively insensitive to shock, thereby making the fuel safe to handle and store under what would ordinarily be regarded as adverse conditions. We accomplish this desirable result by blending nitromethane with nitropropane in such proportions that a self-combustible and relatively non-detonatable fuel is produced. The effectiveness of this fuel is very materially improved by the addition of a suitable catalyst. We have found that the addition of chromium acetonylacetonate as a catalyst is especially effective in assisting the smooth decomposition of the compounded fuel in the motor. Such a fuel will not easily detonate under the conditions of temperature and pressure which exist in a jet motor during operation and if detonation should occur at some point in the system, the detonation will not propagate to the rest of the system.

An added feature of our invention is that the gases are produced at lower temperatures by this fuel and therefore are more suitable for use in a turbine jet motor.

Our invention will be better understood from the following detailed description and the enclosed schematic drawing which shows a jet propulsion system suitable for employing the fuels of this invention.

The drawing shows a jet motor comprising a firing chamber 11 closed at one end and provided with an exhaust nozzle 12 at the other end. The blended fuel and catalyst are placed in tank 13 and are supplied to the firing chamber by conduits 14 through injector 15. A suitable ignition device such as a spark plug 16 is employed to fire the charge. Oxygen from a storage tank 17 is connected to the firing chamber through constant pressure valve 18 and conduit 19. The flow into the chamber through injector 23 is controlled by an electromagnetic valve 27.

Fuel in tank 13 is placed under pressure by some suitable means such as an inert gas pressure tank 21 which supplies constant pressure to the fuel tank through constant pressure valve 23. An electromagnetic throttle valve 22 controls the admission of fuel into the firing chamber.

The inert gas storage tank 21 is also connected to the firing chamber directly through a line 24 provided with a constant pressure valve 25. The flow of gas into the firing chamber through nozzle 28 is controlled by an electromagnetic throttle valve 26.

The fuel employed in the apparatus and which is the subject of this invention is compounded by blending nitromethane with nitropropane. We have found that a selfcombustible relatively non-detonatable fuel may be obtained when the mixture of nitromethane and nitropropane contains 20% to 40% (by volume) of nitropropane. If the nitropropane is less than 20% by volume of the nitromethane-nitropropane mixture, the mixture will be undesirably sensitive to shock and may be detonated under some conditions. However, such a mixture forms a low order of explosion and will not propagate the explosion to other parts of the system through connecting lines. The propagation of the explosion does not begin to take place until less than 15% (by volume) of nitropropane is employed in the nitropropane-nitromethane mixture. Likewise if the proportion of nitropropane exceeds 40% of the nitromethane-nitropropane mixture the fuel becomes deficient in oxygen and is no longer self-combustible. This fact is sometimes evidenced by the formation of carbon during the operation of the motor.

A preferred fuel mixture suitable for operation in a jet motor is one compounded by mixing nitromethane with 20% to 35% nitropropane (by volume). The factors which will determine the actual percentage to be employed are the specific impulse required and the chamber temperature at which it is desired to operate the apparatus. For operation we prefer to employ a mixture of nitromethane and 25% nitropropane (by volume). A form of nitropropane found especially satisfactory was the 2-nitropropane having the formula:

a tutti However, l-nitropropane may also be employed.

We have discovered that decomposition of our nitromethane-nitropropane fuel mixture is materially assisted by the presence of a suitable amount of catalyst such as Cr O This may be formed by introducing into the heated firing chamber an organic chromium compound. The organic chromium compound is preferably dissolved or suspended in the nitroparaflin fuel mixture. An example of this type of compound is chromium acetonylacetonate. The catalyst is blended with the fuel prior to its transfer into the storage supply tank 13. The catalyst is added to the nitroparafiin mixture in amounts varying between 0 and 8% by weight. It has been observed, however, that 3% of the chromium acetonylacetonate are employed in the fuel and successful runs have been made when the amount of catalyst was lowered to as low as one percent. The use of amounts greater than 3% of the catalytic material does not appear to improve the rate of decomposi tion of the nitroparaffins substantially; and apparently the main effect derived in increasing the amount of catalyst above 3% is to add to the cost of the propellant.

To facilitate starting the jet motor when cold and to permit the fuel to begin producing its maximum efiiciency as soon as possible after starting, a stream of oxygen is introduced into the combustion chamber simultaneously with the fuel during the initial firing period. The supply of oxygen may be discontinued after a short period which has been found to vary between 5 and 15 seconds duration. After the motor is smoothly operating it is unnecessary to supply additional oxygen.

The manner of operating the motor with our novel fuel is as follows: The combustion chamber oft-he motor 11 is flushed with an inert gas. supplied from storage tank 21 to eliminate any unburnt materials and oxygen that may be present in the chamber. The fuel which contains the catalyst is drawn from'a tank'13 and intro-- duced into the firing chamber as a mist or fine spray simultaneously with a stream of oxygen from acontainer 17. The fuel in the chamber 11 is ignited by a spark plug 16 and combustion takes place in the presence of excess oxygen for a period of approximately seconds,

starting from the time that the initial charge is ignited; This period serves as a warming up period to further the smooth decomposition of the nitroparafiin fuel in the motor. After this warming period theoxygen supply isdiscontinued and the decomposition of the nitroparatfin progresses smoothly without further additions of oxidizers.

The chamber pressures which should be maintained during the operation of the motor should lie between approximately 350 and 700 p. s. i., and it is preferable to maintain the chamber pressures above 400 p. s. i., since. it has been observed that when the pressure'drops below the minimum value of 350 p. s. i., there is a tendency for thedecomposition to proceed only partially with attendant formation of intermediate decomposition products such as. formaldehyde and other noxious gases. The maintenance. of desired chamber pressures is obtained in a manner well. understood in the art by suitable selection of proportions of the exhaust nozzle and combustion chamber. in relation to the rate of fuel injection. Formation of such toxic gases is not desirableand their presence may impair satisfactory use of the apparatus.

The fuels according to our invention possess the advantage that they are self-combustible propellants and may be practically and etficiently employed in a jet motor without exposing the operator or apparatus tothe danger of explosion and to the attending danger of propagation of the explosion to the main storage supply through the fuel lines.

Another advantage which has been observed is that some blends of our fuels are capable of producing specific impulses which are substantially as high as those obtained when nitromethane alone is employed. It will be recognized, of course, that different motor dimensions and injection rates would produce different impulses. The following examples are illustrative: A 20% nitropropane- 80% nitromethane mixture: (by volume) under ideal conditions created a specific impulse of 210 lbs.sec;'/lb., and a chamber temperature of 2160" K., when injectedata.

definite rate into a particular combustion. chamber equipped with a suitable exhaust nozzle. A mixture containing 40% nitropropane-60% nitromethane (by volume)v when injected at the same rate. into the; same motor developed a specific impulse'of 196 lbs;sec./lb;,.and' generated a chamber temperature of.1780 K. When'nitromethane is employed alone the specific impulse developed inthe motor employed above and under ideal conditions is .215 lbs.sec./lb., and the chamber temperature is 2460 K. These values in actual operation are subject to modification due to variations in design and to experimental losses. I

A further advantage to be derived by the use of our novel fuel is that it will operate underwater apparatuses and since it is a self-combustible propellant it is not necessary to provide for the addition of oxidizer as is the case with a Diesel or hydrocarbon fuel type of combustion engine.

If it is desired to increase the specific impulse or the chamber temperature using the same motor as above and employing the same fuel it is possible to do so by conducting the combustion in an atmosphere con taining varying percentages of added oxygen to the Weight volume) the impulse was increased from 210 lbs.sec./1b.,-

to approximately 240 1bs.sec./lb.

It is apparent that wide ranges of specific impulse and chamber temperatures are made available by employing the mixtures described in our invention and this desirable feature accompanied by relative freedom from detonationopens a new field of application for jet motors which have hitherto been confined to installations where safety was not an important factor.

.A characteristic of the above fuels is that after the motor has once been started and after the chamber pres= sure has been raised sufiiciently high the motor willcontinue to operate with a practically invisible flame escaping from the jet orifice and the discharge will be sub stantially free of smoke and noxious fumes.

We claim:

1. A fuel comprising 20% to 40% by volume of 2- nitropropane dissolved in nitromethane and containing chromium acetonylacetonate.

2'. A fuel comprising 20% to 40% by volume of 2- nitropropane dissolved in nitromethane containing 1% to- 8% chromium acetonylacetonate by weight based on the weight of 2-nitropropane-nitromethane mixture.

3. A fuel comprising 35% by volume Z-nitropropane, by volume nitromethane in which is dissolved about 3% of the weight of chromium acetonylacetonate based on the weight. of the 2-nitropropane-nitromethane mixture".

References Cited in the file of this patent UNITED STATES PATENTS 1 ,673,620 Muller-Cunradi June 12, 192 8 1,820,983 Loomis Sept. 1, 193'1' 2,086,775- Lyons July 13, 1937 2,233;620-' Lippincott Mar. 4; 1941" 2,325,064 Lawrence July 27, 1943" 2325;065 Lawrence July 27, 1943 2355,817 Morrow- Aug. 15, 1944" 2,394,315 Levy Feb; 5,. 1946 2,397,657 Goddard Apr. 2, 1946 2433,932 Stosick Jan. 6'. 1948 2,433,943- Zwicky et a1. Tan. 6, i948 

1. A FUEL COMPRISING 20% TO 40% BY VOLUME OF 2NITROPROPANE DISSOLVED IN NITROMETHANE AND CONTAINING CHROMIUM ACETONYLACETONATE. 